In digital communication systems, wherein, for example, data processing stations are connected to generally accessible communication lines via modems, arrangements are known for inhibiting access to a given data processing station in order to prevent unauthorized reception of data, or unauthorized modification of data in a given station. Such arrangements generally comprise the encoding and decoding of the data signals with various algorithms. While such arrangements provide a degree of data and system security, the codes that are employed have been frequently broken, resulting in loss of valuable information and/or destruction of processing ability of the data station.